1. Field of the Invention
The invention relates to a signal process apparatus for an on-vehicle radar and a method thereof, by which a transmission signal and a reception signal are mixed to produce a beat signal and processing the beat signal enables an object reflecting the transmission signal to be detected.
2. Description of the Related Art
From the past, there has been provided a vehicle space control apparatus for a vehicle that can detect a preceding vehicle and keep proper a distance between the vehicles, and a vehicle space alarm apparatus for a vehicle that can notify a driver of the vehicle of the fact that a distance between the vehicle and another vehicle is shortened more than is considered to be safe. Those apparatus employ, as one of on-vehicle radars, a frequency modulated continuous wave (FMCW) radar (hereinbelow called a xe2x80x9cFMCW radarxe2x80x9d) that uses a millimeter wave region.
The FMCW radar makes use of a radar wave that is modulated so that frequencies of the radar wave can increase or decrease linearly in the form of a triangle with respect to time. Based on a beat signal derived by mixing a transmission signal of the radar wave with a reception signal of the radar wave reflected by an object, the FMCW radar gets information on the object that reflects the radar wave.
Specifically, with respect to each of rising modulation time when the frequency of the radar wave increases and at falling modulation time when the frequency of the radar wave decreases, a frequency analysis process, such as the fast Fourier Transform (FFT), of the beat signal is performed to obtain a power spectrum of the beat signal for each modulation time and peaks of the power spectrum. The peaks derived from the power spectrum for the two modulation times are properly combined to produce a combined peak (hereinbelow called a xe2x80x9cpeak pairxe2x80x9d), which can be applied to a well known calculation formula. The application establishes a distance and a relative speed of the object specified by the peak pair.
FIG. 1 is a diagram for illustrating how an on-vehicle radar in prior art has a problem. FIG. 1(a) is a case where there are roadside objects on the left front side of and a preceding vehicle in front of a predetermined vehicle. FIG. 1(b) is a case where there are two preceding vehicles running in parallel in front of a predetermined vehicle. FIG. 1(c) is a diagram for a power spectrum obtained for FIGS. 1(a) and 1(b) with respect to the frequency BIN (distance). The solid line shows a detected power and the dotted line shows a predetermined threshold for determining whether or not a detected peak should be extracted as a peak. When, as shown in FIG. 1(a), the preceding vehicle, after detected, proceeds to a region where there are the roadside objects, or when, as shown in FIG. 1(b), the preceding vehicle, after detected, runs in parallel with another vehicle, the FMCW radar detects a power spectrum as shown by the solid lines in FIG. 1(c), where a peak (represented by a chain line) based on the detected preceding vehicle is buried in the peaks for the roadside objects or the preceding vehicle, without being detected. Failure to detect a peak is a problem.
There is proposed a method of detecting a buried object. The method uses an array of antennas that receives a radar wave, which produces a phase difference. Using the phase difference in the array, digital beam forming (DBF) process is performed, by which the buried object is separated in a direction to be detected.
The DBF process is carried out specifically as described below. That is, from a power spectrum (called a xe2x80x9cdistance power spectrumxe2x80x9d since frequencies correspond to distance) of a beat signal obtained for each antenna, signal components of the same frequency are extracted, respectively. The frequency analysis process, such as the FFT, of the extracted signal components are executed to get a power spectrum (called a xe2x80x9cdirection power spectrumxe2x80x9d since frequencies correspond to direction). The process of getting the direction power spectrum is performed over an entire range (entire frequency BIN) of the distance power spectrum, which is classified for the same direction to form a beam.
However, since operation load for performing the beam formation is sufficiently large, high speed processors must be used to guarantee a speedy detection of the object. Accordingly, there is a problem that those high speed processors are expensive.
Moreover, Japanese Patent Application Laid-open No. 2001-228239 discloses a technique, by which the peaks of a distance power spectrum are extracted and performed only for the peaks is the operation of the direction power spectrum to reduce operation load.
However, the technique cannot derive a peak buried in another peak whose frequency is different in the distance power spectrum. Therefore, an object that produces the buried peak cannot be detected, which fails to provide a sufficient detection capability. This is also a problem.
As mentioned above, since the prior art gives rise to the problems, there is a need to resolve the problems.
The invention is directed to a signal process apparatus for an on-vehicle radar and a method thereof that satisfy the need. The invention reduces operation load and provides a sufficient detection capability.
One aspect of the invention involves a signal process apparatus for an on-vehicle radar having a plurality of channels formed by a transmission antenna and a reception antenna array, and for processing a beat signal derived by mixing a transmission signal of a radar wave transmitted by the transmission antenna with a reception signal received by reception antenna array the through the channels. The signal process apparatus comprises a first operation unit for performing frequency analysis of the beat signal and obtaining a distance power spectrum for each of the channels; a prediction point setting unit for setting a prediction point where an object is considered to be existent based on information other than peaks of the distance power spectrum; and a second operation unit for obtaining a direction power spectrum at the prediction point established by the prediction point setting unit based on a operation result of the first operation unit, whereby based on the operation result of the first and second operation units, information on the object that reflects the radar wave is obtained.
The signal process apparatus procures information concerning an object that reflects a radar wave, based on operation result by the first operation unit and the second operation unit.
The signal process apparatus for an on-vehicle radar can reduce operation load for detecting an object, since obtained is a direction power spectrum only on a prediction point where the object is considered to be existent, not a direction power spectrum on an entire region of a distance power spectrum obtained by the first operation unit.
Even when there is a peak buried in other peaks of different frequencies on a distance power spectrum, a signal process apparatus does not miss the buried peak to produce a high detection capability, since a prediction point is set based on information other than the distance power spectrum that is an operation result by the first operation unit.
Advantageously, the prediction point setting unit includes a running line estimator for estimating a running line that a predetermined vehicle is predicted to run, and a hidden peak extracting unit for obtaining a power spectrum along the running line that the running line estimator estimates and extracting a peak of the power spectrum, whereby a peak frequency that the hidden peak extracting unit extracts is set as the prediction point.
In the case, an object on a running line is detected preferentially, which is advantageous to an apparatus that controls a distance between a predetermined vehicle and a preceding vehicle (called a xe2x80x9cvehicle distancexe2x80x9d hereinbelow). Advantageously, the running line estimator estimates the running line based on at least one of outputs of a steering angle and a yaw rate sensor of the vehicle.
Advantageously, the running line estimator estimates the running line based on a shape of a road in front of the vehicle specified by an arrangement of stationary objects that has already been detected, and the running line estimator estimates the running line based on a shape of a road in front of the vehicle specified by map information and present position information on the vehicle provided from the outside.
Advantageously, the hidden peak extracting unit obtains a power at each point on the running line by the use of the Discrete Fourier Transform, when a power spectrum is obtained along a running line.
That is, when a power is obtained on each point along a running line, a power at unnecessary points is not calculated by the use of the FFT; instead, a power at necessary points is only calculated by the use of the DFT, which leads to a decrease in operation load.
Advantageously, the prediction point setting unit includes a peak predicting unit for predicting peaks that the detected objects is supposed to produce on the distance power spectrum, based on information obtained during a previous detection cycle on the detected objects, wherein the peak predicting unit establishes as the prediction point each frequency in a predetermined frequency range that contains peak frequencies to be predicted.
In the case, since a peak with respect to the detected object is detected preferentially, a newly produced peak can be differentiated without fail.
Advantageously, the apparatus comprises an extracting unit for extracting a peak, supposing that the peak on the detected objects is detected, when there is a possibility that the peak about the detected objects is buried in another peak by other objects on a directional power spectrum that the second operation unit obtains, with respect to the prediction point that the peak predicting unit has established.
The judgment as to whether or not there is a possibility that a peak is buried is carried out, for example, by the fact that there is a portion for a peak to be predicted whose power is beyond a predetermined threshold and that there is another portion in the vicinity that has more power.
Each constituent element in the signal process apparatus may be a program for performing a function of the constituent element.
In this case, the programs can be stored in a recording medium such as a floppy disc, a mini disk, a digital versatile disc, a compact disc-ROM, a hard disc, and a memory card that computer can read out. The stored programs, if need arise, may be loaded in a computer system to be used. In addition, the programs may be stored as a recording medium readable by a computer in a ROM and a backup RAM, which can be incorporated in a computer system. The programs not only in recording media but also from networks can be loaded for their use.
One aspect of the invention involves a method of processing a beat signal derived by mixing a transmission signal of a radar wave transmitted by a transmission antenna with a reception signal received by a reception antenna array through a plurality of channels in a signal process apparatus for an on-vehicle radar. The method comprises performing frequency analysis of the beat signal and obtaining a distance power spectrum for each of the channels; setting a prediction point where an object is considered to be existent based on information other than peaks of the distance power spectrum; and obtaining a direction power spectrum at the prediction point established by the prediction point setting unit based on a operation result of the first operation unit, whereby based on the operation result of the first and second operation units, information on the object that reflects the radar wave is obtained.
One aspect of the invention involves a method of processing a beat signal derived by mixing a transmission signal of a radar wave with a reception signal from a detected object in a signal process apparatus for an on-vehicle radar having a plurality of channels. The method comprises registering as peak data a frequency BIN contained in a frequency region around a prediction peak that is to be detected on a distance power spectrum, from information on the object; predicting a running line on which a predetermined vehicle runs to obtains a power spectrum along the running line; registering as peak data a peak of the power spectrum; obtaining a distance power spectrum for each channel; averaging the distance power spectrum; registering as peak data a peak of the averaged distance power spectrum; and seeking a directional power spectrum only with regard to the registered peak data.